The invention relates generally to medical devices and, more particularly, to a method and apparatus for detecting whether a load connected to an external defibrillator is a patient or a test device.
Ventricular fibrillation is one of the most common life-threatening medical conditions that occurs with respect to the human heart. A common treatment for ventricular fibrillation is to apply an electric pulse to the heart that is strong enough to stop the heart""s unsynchronized electrical activity and give the heart a chance to reinitiate a synchronized rhythm. External defibrillation refers to a method of applying an electric pulse to a fibrillating heart through the surface of a patient""s body.
When a defibrillation pulse is applied to a patient, the pulse encounters a resistance to the flow of electrical current through the patient. The resistance of a patient""s thorax to the flow of electrical current is called transthoracic impedance (TTI). The magnitude of current flowing through a patient is directly proportional to the magnitude of the voltage difference across the electrodes used to deliver the defibrillation pulse to the patient and inversely proportional to the patient""s TTI.
A patient""s TTI is comprised of two components: resistance and reactance. The conductive characteristic of body fluids provides the resistive component, whereas cell membranes, acting as imperfect capacitors, contribute to the reactive component. An impedance that includes both resistance and reactance is known as a complex impedance.
The measurement of a patient""s TTI varies according to the amplitude of the electric signal applied to the patient. The impedance encountered by a small-amplitude signal applied to a patient is different than the impedance encountered by a high-amplitude signal (e.g., a defibrillation pulse).
Advances in defibrillation technology have found that defibrillation therapy may be optimized by adjusting the magnitude and duration of a defibrillation pulse according to the patients defibrillation impedance (i.e., the impedance encountered by a defibrillation pulse applied to the patient). While an initial defibrillating pulse may be applied to a patient in order to measure the patient""s defibrillation impedance, it is preferable that the initial defibrillation pulse already be optimized according to the patient""s defibrillation impedance. A suitable method for predicting a patient""s defibrillation impedance involves first sending a small-amplitude signal through the electrodes of the defibrillator and measuring the impedance encountered by the small-amplitude signal. A transformation equation is then applied to the small signal impedance measurement to produce a predicted defibrillation impedance of the patient. A transformation equation of this type is typically generated by correlating small signal impedance measurements previously obtained for a population of patients with high-amplitude (i.e., defibrillation) impedance measurements obtained for the same population.
While applying a transformation equation to a small signal impedance measurement is useful in predicting a patient""s defibrillation impedance, a problem arises when testing the defibrillator on a test device, or test load, instead of a patient. A typical test device for testing a defibrillator is comprised of a high-power 50 ohm resistor intended to represent the patient. A defibrillation pulse is discharged from the defibrillator into the test device and measured to ensure that the defibrillator is working properly. Because the resistor in a test device has only a resistive component, and does not present a complex impedance to the defibrillator, the measured impedance of the test device does not vary with the amplitude of the electric signal applied to the device. In other words, a test device""s xe2x80x9cdefibrillation impedancexe2x80x9d will be the same as the measured small signal impedance.
Unless a defibrillator is manually set in a xe2x80x9ctestxe2x80x9d mode of operation, the defibrillator does not know that a test device (as opposed to a patient) is connected to the defibrillator. Consequently, when the defibrillator applies a transformation equation to a small signal impedance measurement of a test device to xe2x80x9cpredictxe2x80x9d the test device""s defibrillation impedance, an incorrect predicted impedance is produced. This incorrect defibrillation impedance is then used to shape the defibrillation waveform that is delivered to the test device, resulting in the wrong amount of energy being delivered to the test device.
Furthermore, a defibrillator typically collects data relating to the delivery of a defibrillation pulse and stores this data in a memory for later review by medical professionals and others. However, a defibrillator""s memory is limited in size and often holds only the most recent data collected by the defibrillator. In a test situation where a test device is attached to the defibrillator, it may be undesirable to store the generated data and overwrite earlier data relating to defibrillation pulses delivered to actual patients.
Accordingly, there is a need for a method and apparatus that can automatically differentiate when a test device, as opposed to a patient, is connected to a defibrillator. The present invention addresses the this need and other shortcomings in the prior art.
A device constructed in accordance with the present invention automatically detects whether a load that is connected (or to be connected) to a defibrillator is a patient or a test device. The device assists in deciding when a transformation equation should be used to predict the defibrillation impedance of the load. The transformation equation is applied to a small signal impedance measurement of the load when the device determines that the load is a patient.
The device of the present invention also assists in deciding when data collected by a defibrillator while connected to a test load should be saved in a memory. If the load connected to a defibrillator is determined to be a test device, the defibrillator may selectively not save data generated while connected to the test device, thus preserving memory space in the defibrillator for data relating to defibrillation therapy delivered to actual patients.
A device constructed according to the present invention may be implemented as a stand-alone device or as an integral part of a defibrillator or other medical device. In one embodiment of the invention, a defibrillator (or other device embodying the invention) utilizes at least two small-amplitude signals having different frequencies to measure at least two load-dependent electrical parameters of the load. Suitable load-dependent electrical parameters include small signal impedance measurements of the load. The small signal impedance measurements are compared to each other to determine the nature of the load.
Because a patient presents a complex impedance to an electric signal (in contrast to a test device that presents only a purely resistive impedance), small signal impedance measurements of a load that is a patient will differ according to the frequencies of the small-amplitude signals used to measure the patient""s impedance. Thus, a load is determined to be a patient if the small signal impedance measurements of the load are not approximately equal.
On the other hand, a test device presents only a resistive impedance to the defibrillator. When measuring the impedance of a test device using small-amplitude signals of different frequencies, there will be little or no appreciable difference in the small signal impedance measurements of the load. Consequently, if the small signal impedance measurements of a load are approximately equal, the defibrillator determines that the load is a test device.
In an alternative embodiment of the invention, a small signal impedance measurement obtained prior to defibrillation may be compared with a high-amplitude signal impedance measurement (e.g., defibrillation magnitude pulse), to determine the nature of the load. If the load is a patient, the impedance measurements will differ. If the load is a test device, the impedance measurements will be approximately equal.
In accordance with one aspect of the invention, if the defibrillator determines that the load is a patient, the defibrillator may automatically apply a transformation equation to one or both (or some combination) of the small signal impedance measurements to predict the high-amplitude defibrillation impedance of the load (i.e., patient). Using a predicted defibrillation impedance of the patient, the defibrillator may adjust the amplitude and duration of the defibrillation pulse to be applied to the patient to compensate for the patient""s particular impedance, and thus increase the effectiveness of the defibrillation pulse to be applied to the patient.
In accordance with another aspect of the invention, if the defibrillator determines that the load is a test device, the defibrillator may proceed to prepare and deliver a defibrillation pulse with amplitude and duration characteristics optimized for the measured small signal impedance of the load without applying a transformation equation as discussed above. The defibrillator may also automatically enter into a test mode or calibration mode of operation. In this mode of operation, the defibrillator may provide an option of not saving data normally collected and stored by the defibrillator while connected to a patient. Alternatively, the defibrillator may be configured to automatically not save such data if the load is determined to be a test device.